Effect of mild oxidation on the surface chemistry of bituminous coals under different humidity conditions

The influence of humidity conditions on the oxidation of four coals has been studied on the basis of their surface chemistry. A systematic study was carried out in which four coals of different rank were oxidised at 50 °C in two conditions of humidity (20 and 90% moisture levels) and in an air atmosphere. The changes in the surface functional groups were measured by means of diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and X-ray photoelectron spectroscopy (XPS). The lowest rank coals were the most affected by oxidation in all the humidity conditions applied. In the case of the higher rank coals, higher moisture levels promoted the oxidation process. The number of oxygen-containing structures increased after oxidation, the most abundant being the carbonyl and carboxyl groups